the point(latLng) is a property of the mouseEvent
google.maps.event.addListener(polylineInstance, 'mousemove',function(e){
console.log(e.latLng)
})
To get the clicked segment of the Polyline, iterate over the path of the Polyline, create a temporarily polyline for each segment and use google.maps.geometry.poly.isLocationOnEdge()
to check if the click has been on the current segment.